1. Field of the Invention
The present invention relates to a plated substrate for hard disk medium and a manufacturing method thereof.
2. Description of the Related Art
Si single crystal substrates have been widely used as semiconductor wafers because of purity and ease of patterning. Furthermore, recently, by taking advantage of excellent characteristics in rigidity, surface smoothness, stability of surface state, etc., utilization thereof as high recording density magnetic recording substrates have also been examined as disclosed in, for example, Japanese Patent Publication (JP-B) No. 1-42048/'89, Japanese Patent Publication (JP-B) No. 2-41089/'90, Japanese Patent Publication (JP-B) No. 2-59523/'90, Japanese Patent Publication (JP-B) No. 1-45140/'89, Japanese Patent Provisional Publication (JP-A) No. 57-105826/'82, Japanese Patent Provisional Publication (JP-A) No. 6-68463/'94, Japanese Patent Provisional Publication (JP-A) No. 6-28655/'94, and Japanese Patent Provisional Publication (JP-A) No. 4-259908/'92.
When an Si single crystal is used as an electronic material or as a magnetic recording material, it is necessary to coat the surface thereof with a metal film for installation of electrodes or application of a magnetic material. Such metal film coating on an Si substrate has been carried out by vapor deposition under vacuum such as sputtering method.
However, in recent years, in LSI manufacturing, because of population of the damascene process, etc., and an application of manufacturing perpendicular magnetic recording media in the field of magnetic recording materials, it has become necessary to provide a thick metal film coating which allows polishing.
Accordingly, various attempts to coat an Si substrate with a metal film have been examined by a plating method, which can easily realize a thick film compared to vapor deposition under vacuum.
In order to provide plating having excellent adhesion by wet-process plating, it is important that a substance which can be a catalyst for reducing metal ions in a plating liquid exists in large quantities in a junction part between a parent material and a plating film. Furthermore, the greatness in adhesion between the formed plating film and plated parent material depends on a mechanical anchoring effect due to a surface unevenness of the plated material or a chemical interaction between the plated material and plating film.
For example, in order to provide plating on the surface of a material low in chemical reactivity such as a plastic, ceramic or glass material, a method has been widely carried out, wherein after roughening the parent material surface by polishing, etc., colloidal particles are firmly fixed to surface concavities by an immersion in a Pd—Sn colloid solution, and plating is carried out by using this colloid adhesion as a catalytic origin so that adhesion resulting from a mechanical anchoring effect can be secured.
On the other hand, in plating on a metal such as Fe, a metallic bond is formed-between the plating film and plated metal immediately after the start, and occurrence of alloying at an atomic layer level secures a strong adhesion.
Si, which is used as a plating parent material, is considerably high in reactivity with respect to oxygen, and in a few hours after manufacturing, its surface is already covered with an SiO2 natural oxide film of a low chemical activity and is passivated. Therefore, it is difficult to form a chemical bond with a plating film.
It is widely known that such a natural oxide film on an Si surface can be resolved and removed by an HF immersion, etc. However, since the Si surface after removal of the natural oxide film is considerably easily oxidized, if it is immersed in a plating liquid, an oxide film is formed by a reaction with OH groups in the liquid before a plating film is formed. Therefore, a satisfactory plating film cannot be obtained.
Accordingly, plating on an Si substrate is carried out by one of the following methods. In a similar manner to the plating on plastic, etc., as mentioned in the foregoing, the substrate is, after roughening the substrate surface, immersed in Pn-Sn colloid for plating. Or after introducing a metal layer by vapor deposition such as a sputtering method, plating is provided on this metal layer.
However, in the method where plating is carried out after roughening the substrate, it becomes necessary to increase the roughness of the substrate surface for a further improvement in adhesion of the plating film. Therefore, this is not preferable for plating on a semiconductor wafer, etc., used for an electronic material, etc. In addition, when the substrate surface is roughened by machining, a problem arises such that machining causes processing traces and, depending on the size and shape of the processing traces, substrate strength may be considerably deteriorated.
On the other hand, when plating is carried out after introducing a metal layer on the Si substrate surface by vapor deposition, in order to carry out counter diffusion between the Si substrate and metal layer at an atomic level, it is necessary to carry out heat annealing during deposition or after deposition. In this case, since metal activation immediately after deposition is lost by thermal annealing, it is necessary to reactivate the deposited metal film by a Pd—Sn catalysis, etc., before plating as a pretreatment. In addition, there exists a problem such that since vapor deposition of dry film formation and wet-process plating are simultaneously employed, the equipment is complicated and increased in size.